1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus and, more particularly, to a fixing device for fixing a toner image on a sheet carrying thereon unfixed toner and an image forming apparatus including the fixing device.
2. Description of Related Art
Conventionally, an image forming apparatus, such as a copier, a printer, a facsimile machine, and a combined machine including their multiple functions, is configured to form an image on a sheet by fixing or fusing a toner image transferred to the sheet by use of a fixing device. The fixing device performs a fixing process to heat the sheet while the sheet is passing through a fixing nip provided between a pair of fixing members, thereby fixing the toner image on the sheet.
Herein, during the fixing process, a surface of the sheet that bears the toner image may be not smoothly separated from the fixing device even after the sheet passes through the fixing nip and may remain stuck to the fixing device, resulting in separation failure. This separation failure is likely to be caused when a large amount of toner is adhered to a leading end side of the sheet.
For example, Japanese unexamined patent application publication No. 2008-107658 discloses an image forming apparatus arranged to perform image formation in such a manner that a toner adhesion degree is compared between a leading end part and a rear end part of a toner image to be transferred onto a sheet and, when an amount of the toner to be transferred to a rear end side (bottom) of the sheet is smaller than that to a leading end side (top), the leading end part and the rear end part of the toner image are inverted. This can transfer either part of the toner image with a smaller toner adhesion amount onto the leading end side of the sheet, so that separation failure can be suppressed.
However, the conventional apparatus configured as above to suppress the failure to separate a sheet from the fixing device could cause an adverse result on an image formed on the sheet. Specifically, some images remaining inverted top to bottom may be output by the image forming apparatus. If the orientation of images is different from sheet to sheet output by the image forming apparatus, a user has to arrange the sheets so that their images are aligned in the same orientation. This is a troublesome task for the user.
The present invention has been made to solve the foregoing problems of the conventional techniques. Specifically, the present invention has objects of providing a fixing device and an image forming apparatus, in which a sheet having passed through a fixing nip can be appropriately separated from the fixing device without exerting any influence on an image to be formed.